In my experience, the mails that refuse to display anything but "please view this message in your browser" are never mails I actually want to look at.
I'm not arguing that you have to be willing to put up with some friction! But to me it's not that bad.
[edit: typo]
Viewing html email in emacs is usually pretty easy. The html2text is usually good enough.
The real problem is sending email. Plain text email looks like trash in any modern mail reader. Any recipient will wonder why your email is broken. On a desktop client the lines will be way too short. On a phone, too long.
Format flowed or whatever doesn't solve it. The lines just need to be html paragraphs.
I saw a blog post about an mu4e plugin that would allow you to write your email in markdown, and it will htmlify it for you. That would be a big help, assuming you can preview before sending. It is easy to make markdown errors without a preview.
I disagree on "too short" lines being a problem for desktop, but in any case, it is quite possible to send plaintext mail without 70-column line breaks. How I do so in VM in Emacs: https://lists.nongnu.org/archive/html/viewmail-info/2020-05/...
I've had to stop 25 years of muscle memory of M-q while writing a message, but otherwise the solution seems quite satisfactory.
Perhaps this?
https://www.reddit.com/r/emacs/comments/g6ujzs/sending_richt...
Would be wonderful if this was incorporated into mu4e.
As for html2text, there's also the Python-based html2markdown, which I prefer.
mu4e-html2text-command "html2markdown | grep -v ' _place_holder;'"
I love being able to write my own Elisp code to have VM work exactly the way I want it.